A variety of metallic alloys which exhibit a shape-memory property are known in the art. Examples of such metallic alloys include Ti-Ni, Ni-Al, Ag-Cd, Au-Cd and the like. Of these alloys, Ti-Ni alloy, exhibiting good mechanical properties and chemical stability, has been investigated for its medical applications as a biocompatible functional material. The applications for this metallic alloy include staples for joining bone and for hollow organ anastomosis, wherein the metallic alloy product is capable of recovering a memorized shape on heating by the body temperature and of retaining a constantly stable strength. A device for preventing blood vessel obstruction is disclosed in Japanese Patent Application (Laid-Open) No. 89859/1982; a medical capsule is disclosed in No. 163309/1982; a blood vessel expander is disclosed in No. 82976/1987, and an artificial blood vessel with rings is disclosed in No. 220648/1987. Furthermore, an application for a clip for cerebral aneurysm has been proposed.
In addition to the metallic alloys, it is also known that certain polymeric materials exhibit the property of shape memory. As for polymers with a shape-memory property, an invention related to norbornene polymers is disclosed in Japanese Patent Application (Laid-Open) No. 53528/1984 (corresponding to European Patent No. 105,775.)
One of the major problems associated with the prior art shape memory metallic alloys and polymers, at least in terms of their use in medical applications, is that they are nonbiodegradable. Therefore, the products formed from the shaped molded materials remain in the patient's body, even after the desired results have been achieved. Often, a second operation or other medical procedure is needed to remove the used materials, exposing the patient to increased risks of complication and adding to the cost of the overall procedure.
Accordingly, it is one object of the present invention to provide polymeric materials which exhibit a shape memory property and are also biodegradable.
Another object of the present invention is to provide molded products formed from the shape memory polymers of the present invention which are useful in medical applications.
These and other objects and advantages of the present invention will become apparent from the following specification.